Ongoing efforts to map susceptibility genes for Bipolar I Disorder (BD 1) have suggested complex interactions between several (unknown) genetic and environmental factors. Most such studies have been conducted among Caucasians. Complementary investigations of other ethnic groups may yield useful insights, especially if unusual patterns of inheritance are observed. Our preliminary studies have suggested that rates of parental consanguinity among BD1 patients in Egypt are elevated twofold in comparison with several sets of controls. To investigate these results rigorously, we propose a systematically designed prospective study, with careful diagnostic procedures and accurate estimation of consanguinity. We will examine rates of consanguinity among parents of a representative sample of Egyptian patients with BD1 (n = 100). They will be compared with parents of matched patients without psychotic disorders. Levels of consanguinity will be evaluated using family history data as well as highly informative genome wide DNA polymorphisms. If the risk for BD1 varies with levels of inbreeding in Egypt, then at least some of the susceptibility loci are likely to act recessively. Then, the marker information may also enable homozygosity mapping studies.